This invention relates generally to the manufacture of television picture tubes and particularly to a system and method for continuously controlling the specific gravity and viscosity of the phosphor slurry applied to the faceplates of such tubes.
During the production of television picture tubes, a phosphor slurry is added to the inside of the faceplate panel. The slurry consists of phosphor crystals suspended in a solution of polyvinyl alcohol (PVA), water and other stabilizing and wetting agents. In order to insure that the entire faceplate is coated with the slurry, excess slurry is applied to the faceplate. The faceplate is rotated while the slurry is dispensed and after the entire faceplate surface is coated, the excess slurry is deposited in a salvage tank and returned to the source from which the slurry was originally dispensed. The specific gravity and viscosity of the salvaged slurry are substantially lower than those of the original supply because phosphor crystals adhere to the faceplate.
Consistency of tube quality requires that the specific gravity and viscosity of the slurry dispensed to every faceplate be within specified limits, and preferably should remain substantially constant. In the prior art, efforts are made to maintain these parameters within the required specifications by replenishing the contents of the dispensing vessel from a nearby filler vessel. The specific gravity and viscosity of the slurry in the filler vessel are above those required for dispensation upon the faceplate and accordingly, when combined with the salvage, the slurry composition hopefully remains within an acceptable range of limits. However, because the parameters are not continuously monitored and properly controlled, substantial variations in the specific gravity and viscosity of the slurry frequently occur.
The instant invention overcomes this difficulty by the provision of a system for controlling the specific gravity and viscosity of the phosphor slurry on a continuous basis.